Liquid crystal display devices have become widely used not only as displays for televisions, computers or portable telephones, but also as in-vehicle displays for car navigation systems or back-seat entertainment devices. Because such liquid crystal display devices output a certain polarized light from inside the device and provide specific polarized light to an observer, when the observer looks at a screen while wearing polarized sunglasses, polarized light from the liquid crystal screen may be shielded by the polarized sunglasses. Depending on the observer's posture, the observer may therefore be unable to observe the screen correctly. Because cases where a passenger in a vehicle wears polarized glasses are relatively numerous, the above problem of lack of visibility of the screen due to polarized glasses, especially regarding in-vehicle displays, has gathered attention.
In this regard, a technology in which a λ/4 plate is disposed toward a panel front side of a front polarization plate of a liquid crystal display device, and linearly polarized light is turned into circularly polarized light and reaches the polarized sunglasses, as disclosed in Patent Literatures 1 to 3, is conventionally adopted.
FIG. 5 illustrates factors causing light from a liquid crystal screen to be blocked by polarized sunglasses.
A backlight light L0 is changed into linearly polarized light by a rear polarization plate 101, and is introduced into a liquid crystal panel 102. The liquid crystal panel 102 typically uses TN liquid crystal cells (twisted nematic liquid crystal cells). Directions of both alignment axes 102a and 102b of liquid crystal particles crossing each other perpendicularly is set in order to have a polarization direction form a 45° angle clockwise with a horizontal line of the screen after optical rotation. Accordingly, a transmission axis 101a of the rear polarization plate 101 crosses perpendicularly the polarization direction after optical rotation. Display light emitted from the liquid crystal panel 102 passes through a front polarization plate 103. A transmission axis 103a of the front polarization plate 103 is parallel to the polarization direction after optical rotation. Light transmitted through the front polarization plate passes through a surface treatment film 104 constituted by a low reflection layer or the like. In this way, light outputted from the liquid crystal screen reaches the polarized sunglasses 105. A transmission axis 105a of the polarized sunglasses 105 is disposed so as to be in a vertical direction when the polarized glasses are worn normally. Thus, among components of reaching light L101 from the liquid crystal screen, a component parallel to the transmission axis 105a is perceived by the observer as transmitted light L101a. 
However, in a situation where the observer wearing the polarized sunglasses 105 inclined his/her head to see an image on the liquid crystal screen, the reaching light L102 is screened off by the polarized sunglasses 105 and does not reach the observer if a polarization direction of reaching light L102 toward the polarized sunglasses 105 is perpendicular or substantially perpendicular to a transmission axis 105a. For example, in case of a liquid crystal display device using the TN liquid crystal cells, the transmission axis 105a of the polarized sunglasses 105 is perpendicular to the polarized direction of the reaching light L102 when the observer sees the screen by inclining his/her head to the right at a 45° angle.
In contrast, as shown in FIG. 6, when disposing a λ/4 plate 107 on a front side of the front polarization plate 103, straight polarized light transmitted through the rear polarization plate 103 is converted into circularly polarized light. In such a case, because reaching light L103, which reaches to the polarized sunglasses 105, always equally includes a component parallel to the transmission axis 105a of the polarized glasses 105 regardless of whether the observer's head is inclined or not, the reaching light L103 passes through the polarized sunglasses 105 and is perceived by the observer as transmitted light L103a. 